With the advent of optical waveguides for use in the communications industry much emphasis has recently been placed on vapor deposition as a materials forming technique. In constructing preforms from which optical fibers may be drawn, vapors of materials such as SiCl.sub.4, GeCl.sub.4 and POCl.sub.3 must be precisely delivered at controlled mass flow rates to the preform construction site where they are reacted and deposited on or in a support. This can be done by passing carrier gases such as H.sub.2, He, N.sub.2, O.sub.2, or Ar through a supply of the material in liquid form to the deposition site as a mixture with the vapors entrained with the carrier gas. In preforming this operation a vaporizer is ordinarily use of the type known as a bubbler which has a carrier gas intake conduit that terminates with an outlet orifice located below the surface of the liquid materials and an outlet conduit extending from the space above the surface of the liquid within the bubbler to the deposition site.
To construct an optical waveguide preform properly the mass flow rate of the vapor must be carefully programmed and accurately controlled. Heretofore control has been achieved with vaporizer controllers such as the Source Vaporizer Controllers sold by the Tylan Corporation of Torrance, California. Controllers of this type employ a carrier gas mass flow rate sensor and a vapor to carrier gas ratio sensor.
The carrier gas flow rate sensor operates on the theory that the heat added to a known mass of gas is proportional to its temperature rise at relatively constant pressure. It employs two resistance heating elements which are part of a bridge circuit, positioned in series with each other on the outside of a sensor tube. Gas is passed through the tube which creates a bridge imbalance, the signal from which is proportional to the mass flow rate. The vapor to carrier gas ratio sensor also operates as a function of heat transfer. This sensor employs one electrical resistance element located in the carrier gas intake stream and another in the vapor and carrier gas stream, hereinafter termed "vapor stream". Again the sensors are elements of a bridge circuit which indicates an imbalance as soon as the properties of the gas and vapor stream differ. This difference is proportional to the ratio of source to carrier gas.
With these controllers the electrical signals from the carrier gas flow rate and vapor to carrier gas ratio bridge circuits are electronically multiplied and the product compared with a preselected set point for vapor mass flow rate. An error signal is then fed through an amplifier to an electrically controlled valve located in line with the carrier gas intake conduit. When an insufficient mass flow is detected the valve in the carrier gas intake conduit is opened further to increase the flow of carrier gas into the vaporizer which, in theory, serves to pick up more vapor and increase the mass flow rate. Conversely, if too great of mass flow rate is detected the valve is closed somewhat. A more detail description of such vaporizer controllers and their components may be had by reference to U.S. Pat. Nos. 3,938,384 and 3,939,858.
Though the just described system and method for controlling vapor delivery has been found to be the best available, it nevertheless is quite inaccurate with deviations from optimum set points ranging as great at 30% over both long and short terms. This is attributable at least in part to the fact that this method assumes a steady state condition of vapor pressure. In actuality however the system is not in a steady state since vapor pressure depends on numerous criteria such as carrier gas retention time within the liquid, the depth at which the bubbles are released within the liquid, total pressure, carrier gas temperature, localized temperature inhomogentities surrounding the bubbles as they travel toward the liquid surface, and heat flow into the bubbler from its environment. These effects all become more important as flow rates increase and the liquid levels in the bubbler decreases since retention by carrier gases also decreases as localized cooling takes place.
It is thus seen that control of the flow rate of carrier gas into a vaporizer is only a relatively crude method of controlling mass flow rate of vapor from the vaporizer because of the dynamics of such systems. Some investigators have sought to overcome this problem by placing an array of temperature sensors in the liquid housed within the bubbler and controlling heat into the bubbler responsive to sensed temperatures. This approach however has also failed to produce as high a degree of accuracy which again is belived to be attributable at least in part to the dynamics of the system.